Waste plastic from municipal and industrial sources is continuously increasing. In the U.S., the amount of plastic waste produced in 2010 was 31 million tons, an increase of 25% in ten years. Only 8 percent of the total plastic waste generated in 2010 was recovered for recycling. Waste plastic represents a considerable part of municipal wastes. Over 78 wt % of the total municipal plastic waste comprises thermoplastics with the remainder being thermosets. Thermoplastics are composed of polyolefins which include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC). These polyolefins can be recycled. A large market exists for municipal and industrial thermoplastic to recover hydrocarbon fuels from these polymers. In particular, PE and in particular high density polyethylene (HDPE), low density polyethylene (LDPE) and PP play an increasing role in energy recovery systems because of their high energy value. These waste plastics are attractive materials for the conversion into liquid fuel and recycled feedstock because of their high recovery conversion rate.
There is a steadily increasing demand for technologies which are capable of converting waste plastic materials efficiently into useful products, and in particular converting non-recycled plastics into liquid fuel using efficient and economical processes based on thermal decomposition.
The art is replete with examples of processes and apparatus for the thermal decomposition of waste plastics.
Many methods disclose thermal cracking of waste plastic feedstock, the majority of these processes use a combination of high temperature and catalysts. U.S. Pat. No. 6,270,630 to Li Xing discloses a 2-step thermal cracking process whereby a first cracking reaction is performed in a rotary vessel at 400-500° C. and a second catalytic cracking step at 600-800° C. using a zeolite catalyst. The 2-step cracking process requires high capital investment and high operating cost. Another disadvantage is the use of a catalyst and the inherent catalyst deactivation over time by coke formed during the high temperature of the second cracking reaction. Furthermore, the cracking reaction is prone to coke formation which reduces the hydrocarbon formation rendering the 2-step cracking process less efficient for making high grade hydrocarbon oil from waste plastic. A common problem with thermal decomposition of waste plastic is the coke formation in the reaction vessel which reduces the heat conductivity. U.S. Pat. No. 6,172,275 assigned to Kabushiki Kaisha Toshiba discloses a method and apparatus for pyrolytical decomposition of waste plastic that includes halogen-containing polymers. The process discloses the use of a liquid heat transfer medium that is mixed with the waste plastics to improve the heat conductivity and thus reduces coke formation by the decomposition reaction. Liquid heat transfer media suffer from increasing the heat conductivity by only small increments that may not reduce significantly the coke formation. Another drawback for using a liquid heat transfer medium is the consumption during the high pyrolysis temperature which requires substitution of liquid heat transfer medium over time with make-up material.
The efficiency of the waste plastic pyrolysis process depends greatly on the residence time of the waste plastic and the fast removal of the hydrocarbon vapor. The international published patent application WO2012/172527 discloses a continuous thermo-catalytic process for decomposing waste plastic in a rotary pyrolysis reaction along with a continuous removal of pyrolysis byproducts from the pyrolysis reactor. The waste plastic feedstock is fed via a screw conveyor into the rotary pyrolysis reactor containing a partition structure located in the center of the pyrolysis reactor. The pyrolysis vapors are removed at the output end of the reactor. As a result, the hydrocarbon vapors formed are subjected to the high pyrolysis temperature along the entire length of the reactor leading to an increase in residence time of the hydrocarbon vapors. Furthermore, the hydrocarbon vapors are subjected to extended exposure of high temperature which in turn produces more non-condensable hydrocarbon gas and thus lowers the value of hydrocarbon fuel.
Pyrolysis processes for waste plastic materials disclosed in the prior art, e.g. U.S. Pat. No. 6,774,271 feed the shredded waste plastics at ambient into the pyrolysis reactor. The time until the waste plastic reaches the decomposition temperature requires more energy, increases the residence time and renders the process less efficient reducing the yield for the desired hydrocarbon oil including the middle distillate and light-range hydrocarbons.
In view of the foregoing, it is apparent that a need exists for a process and apparatus which is capable of safely, economically and continuously producing high quality hydrocarbon oil from a wide variety of waste plastic feedstock.